1. Field of the Invention
The present invention relates generally to concrete finishing machines and, more particularly, to riding concrete trowels having engine droop control.
2. Discussion of the Related Art
A variety of machines are available for smoothing wet and partially cured concrete. These machines range from simple hand trowels, to walk-behind trowels, to self-propelled riding trowels. Regardless of the mode of operation of such trowels, the powered trowels generally include one or more rotors that rotate relative to the concrete surface. Riding finishing trowels can generally finish large sections of concrete more rapidly and efficiently than manually pushed or guided hand-held or walk behind finishing trowels.
Riding concrete finishing trowels typically include a frame having a cage that generally encloses two, and sometimes three or more, rotor assemblies. Each rotor assembly includes a driven vertical shaft and a plurality of trowel blades mounted on and extending radially outwardly from the bottom end of the driven shaft. The driven shafts of the rotor assemblies are driven to rotate at a commanded speed. The machine is steered by tilting one or more of the rotor assembles side-to-side to move the machine forward or reverse or fore-to-aft to propel the machine to the left or to the right. The pitch or flatness of the blades can also be adjusted to adjust the machine's finishing characteristics.
Trowels traditionally were powered by a gearbox mechanically coupled to an internal combustion engine and were steered manually using a lever assembly coupled to the gearbox assemblies by linkage assemblies. More recently, larger trowels have been introduced that are potentially fatiguing to steer manually. These trowels are steered via electrically or hydraulically powered actuators responsive to operator manipulation of joysticks. Some of the hydraulically steered trowels are also powered hydraulically via a hydrostatic drive system powered by the machine's internal combustion engine. The engine is driven at full throttle whenever the rotors are being driven, and rotor speed is adjusted by proportional control of the hydrostatic drive system. Specifically, a foot pedal or similar input device allows the operator to input a commanded rotational speed for the rotor assemblies. A controller provides command signals to a proportional control valve of the hydrostatic drive system based on the foot pedal position to adjust the output control of a variable output hydraulic pump to rotate the rotor assemblies at the operator-desired rotational speed. Operators typically operate the machine at full rotor speed through the vast majority of the machine's operational cycle.
The frictional load between the finishing blades and the concrete surface will vary continuously with concrete curing time, concrete mix, temperature and other ambient conditions, such as humidity. Therefore, as the concrete conditions change, the load placed on the engine will also change. For instance, the load placed on the engine can be much higher for wetter concrete, especially if the pitch of the finishing blades is not appropriate, e.g., is too steep. As the load on the engine increases, it is not uncommon for the operator to continue to demand maximum or full rotor speed notwithstanding the fact that the power being required of the engine is greater than the engine can provide. As a result, the increased load placed on the engine causes the engine to slow down, resulting in a noticeable reduction in power and rotor speed. An operator's natural response to such a decrease is to decrease the foot pedal further, if possible, to increase the rotor speed. Such an increase in demand will impose still more load on the already-overloaded engine. Whether or not additional power output is demanded, the overloaded engine may continue to slow and, in some cases, stall if the operator does not reduce the demand placed on the engine by letting up on the pedal. Additionally, exposing the engine to overloaded conditions over extended periods of time can reduce the engine life.
Accordingly, there is a need in the art to reduce engine overloading in hydraulically powered rotary trowels.
One proposed solution uses a drive motor pressure monitoring valve that monitors the pressure in a selected drive motor, e.g., the most downstream motor. In this proposed solution, the pressure in the selected drive motor is taken as indication of motor torque and, thus, as an indication of the demand being placed on the engine by the hydrostatic drive system. If the motor torque, as measured by the pressure monitoring valve, exceeds a desired torque, a relief valve is actuated to cut or decrease the input control pressure on a pilot pressure circuit in order to reduce rotor speed and reduce the load on the engine. It has been found that this proposed solution is unduly sensitive to system parameters such as motor efficiency and relief valve setting. The system may “hunt” or continuously and rapidly cycle between full-rotor-speed and reduced speed. Moreover, the proposed solution was found to display undesirable rotor performance during high load conditions, such as rotor stalling or an unacceptable decrease in engine speed.
Another drawback of this proposed solution is that since the relief valve is actuated based on a “threshold pressure”, an increase in applied torque is not possible once the relief valve is actuated. In other words, the pressure in the load circuit is a direct indication of the frictional torque demand on the concrete. Therefore, when the pressure threshold is reached, the available torque applied is at a maximum and additional torque is not available.